The invention relates to electrooptical displays and more particularly to active matrix type electrooptical displays with redundant means to provide for substantially complete relief from defective circuits of display elements, also referred to in the art as picture elements or pixels. The built-in redundancy provides a means to correct for defective display element circuits in the fabricated display thereby increasing their manufacturing yield.
Conventional electrooptical displays of the so called active matrix type, particularly of the switching device type, are designed with a redundancy system relative to the electrical isolation elements employed in addressing individual display elements in the display, such as disclosed in an article of Ogura et al., entitled "Active Matrix Color LCD Fabrication by Using Redundancy and Repair System", Japan Display 86, pp. 208-211 (1986). As an example, FIG. 1 herein discloses a circuit diagram which is a representative example of a liquid crystal display (LCD) element circuit employing thin film transistors (TFTs) as such isolation elements and for purposes of redundancy, as shown and demonstrated in the Ogura et al. article. As defined in the art, an isolation element is an electrically active or passive device that enhances the ability of a single display element to be selected or addressed without activating or otherwise affecting other non-selected or non-addressed display elements positioned on same addressing signal and scan lines. These elements can be categorized either as threshold devices, such as diodes, thin film diodes or MIM structures or as switching devices, such as TFTs. Particular concern here is the use of isolation elements in the form of switching devices.
In the particular FIG. 1 illustration, the active matrix type display comprises a plurality of display element circuits each having two TFTs 11 and 12 disposed at the intersection of the orthogonal electrical address or control lines, i.e., between the source, signal or data line, X.sub.m, and scan or gate line, Y.sub.n, and with their sources connected to signal line, X.sub.m, and their gates connected to scan line, Y.sub.n, and their drains connected at point P to the driving electrode of display element 20. Display element 20 comprises a driving or display electrode and opposing or counter electrode between which is disposed a light influencing means. As defined in the art, a light influencing means is a material that emits light, e.g., a gas plasma, or varies in optical properties, such as, intensity, phase or polarization relative to either being reflected from or transmitted through the material, e.g., liquid crystal material. An image signal, via signal line, X.sub.m, is addressed to display element 20 via scan line, Y.sub.n, and stored in the capacitance of liquid crystal material at display element 20.
The employment of a redundancy system in electrooptical displays has become an important component in the manufacture of these displays since their yield is greatly increased. Since an active matrix type electrooptical display contains several tens of thousands to several millions of thin film switching elements which are disposed in a relatively large area to form the display, it is extremely difficult to produce such a display without any circuit defects. In the case where there is no redundancy in the display isolation elements employed in an isolation scheme, any single inoperative isolation element will result in an inoperative display element, thereby reducing the total possible display yield. In such a situation, virtually 100% of the isolation elements must be operative in order to obtain a useful display device. However, with the use of double active isolation elements, such as TFTs 1 and 2, the yield of such displays can be increased. This increase in yield is accomplished by disconnecting a defective TFT with the remaining TFT being operative as an isolation element for operation of the display element. There is an exceedingly very low probability that both of these isolating elements adjacent to each other would both become defective. Therefore, if the redundancy system according to the embodiment shown in FIG. 1 is adopted, it is possible to relieve a defective display element by electrically cutting off or otherwise isolating a defective isolation element.
As proposed by Ogura et al, if one of the two TFTs is defective at a display element, it is isolated by laser trimming. Thus, if the remaining TFT isolation element operates properly, it provides an operable display element without any defect. However, in reality, complete relief from a circuit defect is not accomplished because the removal of a defective TFT causes a corresponding change in the operational voltage characteristics in the corrected display element and its circuit. Thus, a corrected display element will not functionally operate as the same voltage level at a normal, uncorrected display element thereby resulting in different operational voltage levels for the two types of elements. Since the applied voltage level on all signal lines is the same, the intensity between these two types of display elements will be different and the resulting contrast across the electrooptical display will not be uniform.
The difference in the operational voltage characteristics between these two types of display elements may be explained as follows. The sizes of gate-drain capacitances 13 and 14 of TFTs 11 and 12 in FIG. 1 may be designated as C.sub.3 and C.sub.4, respectively, and the size of the liquid crystal capacitance may be designated as C.sub.0. When the scan line Y.sub.n changes from a select or addressed state to a non-select non-addressed state, the potential at the point, P, i.e., the potential at the driving electrode of display element 20, shifts by a voltage expressed as follows: EQU .DELTA.V.sub.0 =V.sub.G x(c.sub.3 +C.sub.4)/C.sub.3 +C.sub.4 C.sub.0),
wherein V.sub.G is an amount of change in the potential at scan line Y.sub.n.
On the other hand, when TFT 12 is found to be defective, such as due to an open circuit or a short circuit eliminated by laser trimming, for example, and, consequently, the display element 20 is driven only by TFT 11 by itself, the shift voltage is changed and is expressed as follows: EQU .DELTA.V.sub.1 =V.sub.G xC.sub.3 /(C.sub.3 +C.sub.0).
Since V.sub.0 .noteq.V.sub.1, there is a difference in voltage that is applied to display element 20 between the two different exemplified circumstances, i.e., in the case where display element 20 is driven by two TFTs 11 and 12 provided for redundancy purposes and in the case where display element 20 is driven by one TFT 11 or 12 due to a defect in the other TFT. As a result, a marked difference occurs in the transmission factor, i.e., there is a visible difference in the light transmission quality or opaqueness of defective display elements compared to normal display elements that have no defect. This marked difference is particularly noticeable in the case of a halftone LC display. Accordingly, the defective display element is not completely relieved of its defective state even with the use of a redundancy system such as illustrated in FIG. 1 since difference voltage level characteristics between a defective display element circuit relieved of its defective isolation element and a normal display element circuit requiring no correction will result in different operational voltage levels resulting in different display element intensities and, correspondingly, result in a different overall contrast across display. Further, there is no simple way to correct the applied voltages to defective display element circuits since the electrical address lines are the same in the case of all display element circuits and the voltage level on signal lines X.sub.m will be the same in all cases for addressed display element circuits.
Thus, it is a primary object of this invention to provide a redundancy system which is capable of substantially complete relief of display element circuit defects wherein no voltage difference results due to different display elements having one, two or even more redundant isolation elements operative to drive the matrix of display elements of the electrooptical display.